Neurotransmitter and neuromodulator release takes place by exocytosis; the influx of calcium into the cell or nerve terminal triggers the fusion of the storage granule with the cell plasma membrane. The membrane fusion events can be modelled by studying the fusion of artificial or biological membranes with each other. Our previous stopped-flow mixing studies have shown that the kinetics of aggregation of small vesicular structures (artificial lipid vesicles, neurotransmitter storage granules, etc.) can be described as the sum of at least two bimolecular rate reactions. A new multichannel, computer controlled stopped-flow rapid mixing spectrometer has been constructed to study the kinetics of these reactions. Using stopped-flow mixing and our new fluorescence assay for fusion, we have extended this work to investigate the fusion of these particles. Small and large unilamellar vesicles composed of phosphatidylserine: phosphatidylethanolamine (l:l) rapid-mixed with calcium show identical aggregation and fusion rates, demonstrating that the rate-limiting step for fusion of these vesicles is aggregation itself. Small unilamellar vesicles with a high radius of curvature leak profusely during fusion while larger vesicles with less radical changes in surface curvature do not. We ascribe this to defects in the packing structure of the membrane phospholipids. This is supported by results from a stopped-flow study of cobalt ion transport across these membranes. Various protein and polypeptides can catalyse fusion of artificial and biological membranes. Some of these have known functions in biological systems, e.g., the spike proteins from rhabdoviruses; therefore in vitro studies of these fusion mechanisms may have clinical relevance. Polylysine will fuse small unicamellar vesicles under conditions similar to spike protein-mediated virus/cell membrane fusion. Stopped-flow studies indicated that polylysine-mediated fusion is not aggregation rate limited and resembles that seen for in vitro fusion of chromaffin granules.